This research is directed towards the clinical problem of thrombosis which is an important cause of morbidity and mortality. Long term aims are to obtain a quantitative understanding of the roles of the different activation and inhibition mechanisms in causing thrombosis, intravascular coagulation and hemostasis. By so doing, it is hoped to facilitate the development of therapeutic methods for thrombotic disease. The immediate goal of the work is to develop blood tests which will permit specific measurement of clotting, thrombotic and fibrinolytic activity in patients. The tests will be principally based on study of the protein fibrinogen. The reasons for selecting this protein for study depend on its clear involvement in clinical disorders of thrombosis, coagulation and hemostasis and the relatively large concentration of the protein in the blood. It is our judgment that biochemical information about the protein and conceptual and technical advance in immunochemistry have proceeded to the point where precise quantitative study can be made of disease related changes in the chemistry and metabolism of fibrinogen. Tests for platelet reaction in vivo are also being developed. An attempt will be made to provide answers to the clinical and physiological problems by developing radioimmunoassays for specific parts of the fibrinogen and fibrin molecules and applying the tests to clinical investigation. The combined efforts of research workers in blood coagulation, protein chemistry, immunochemistry and pathology will be brought to bear on these problems.